It's entirely possible that either your firm or your insurance choice will still take you if you miss your offer grades, so getting one or two grades lower doesn't necessarily mean you'll go straight into clearing. That said, though, it'll obviously make life (or results day, at least) easier for you if you do meet the grades! My best advice would be to do as many past papers or sample questions as possible, mark them honestly (or preferably get someone else to mark them) and improve on your answers using the mark scheme. Your exam technique is just as important as your actual knowledge of the content, so you want to be practicing that as much as you can.